The invention relates to a biaxially oriented polyester film with a base layer composed of at least 70% by weight of a thermoplastic polyester and comprising at least one UV stabilizer and one flame retardant, and with at least one matt outer layer which comprises a mixture or, respectively, a blend made from two components I and II. The invention further relates to the use of the film and to a process for its production.
Component I of the mixture or blend is a polyethylene terephthalate homopolymer or polyethylene terephthalate copolymer, or a mixture of polyethylene terephthalate homo- or copolymers.
Component II of the mixture or blend is a polyethylene terephthalate copolymer which is composed of the condensation product of the following monomers or, respectively, of their derivatives capable of forming polyesters: isophthalic acid, aliphatic dicarboxylic acid, sulfomonomer which contains a metal sulfonate group on the aromatic moiety of an aromatic dicarboxylic acid, and aliphatic or cycloaliphatic glycol.
The outer layer of the invention has a characteristic matt surface or appearance and is very suitable for use in constructing exhibition stands, in greenhouses, or for industrial applications, in particular where a requirement is UV resistance or impermeability to UV light and flame retardancy to DIN 4102 Part 2, construction materials class B2 and B1.
There is high industrial demand for transparent, high-gloss plastic films, e.g. biaxially oriented propylene films or biaxially oriented polyester films. There is also an increasing demand for transparent films of this type which are flame retardant to DIN 4102 and which have at least one surface layer which is not high-gloss but has a characteristic matt appearance and therefore, for example, provides the application with an appearance which is particularly attractive and therefore effective for promotional purposes, and provides protection from UV radiation while also providing flame retardancy.
U.S. Pat. No. 4,399,179 describes a coextruded biaxially oriented polyester film which is composed of a transparent base layer and of at least one matt layer which essentially consists of a certain polyethylene terephthalate copolymer and also comprises inert particles with a diameter of from 0.3 to 20 xcexcm at a concentration of from 3 to 40%. The specific copolymer is a processing aid which reduces the viscosity of the melt comprising the inert particles, thus permitting satisfactory extrusion of that layer. The mattness of the film is achieved by adding the inert particles to the appropriate layer.
EP 0 144 978 describes a self-supporting oriented film made from thermoplastic and, on at least one of its two surfaces, bearing a continuous polyester coating which is applied as aqueous dispersion to the film prior to the final stretching step. The polyester coating is composed of a condensation product of various monomers which are capable of forming polyesters, such as isophthalic acid, aliphatic dicarboxylic acid, sulfomonomers, and aliphatic or cycloaliphatic glycols.
EP-A-0 620 245 describes films with improved heat resistance. These films comprise antioxidants which are suitable for scavenging free radicals formed in the film and degrading any peroxide formed. However, that specification gives no proposal as to how the UV resistance of films of that type might be improved.
DE-A 2346 787 describes a flame-retardant, phospholane-modified polymer. Besides the polymer, the use of the polymer to give films and fibers is also claimed. When films were produced from this phospholane-modified polymer the following shortcomings were apparent:
The polymer is very susceptible to hydrolysis and has to be very effectively predried. When the polymer is dried with prior art dryers it cakes, and production of a film is therefore possible only under very difficult conditions.
The films produced, under extreme and uneconomic conditions, also embrittle at high temperatures. The associated decline in mechanical properties is so severe as to make the film unusable. This embrittlement occurs after as little as 48 hours of exposure to heat.
The instances described give no indication as to how at least one surface of the film can be provided with low gloss while retaining high film transparency, or how the film is to absorb UV light, or how the film is to be provided with high UV resistance and also flame retardancy.
It was therefore an object of the present invention to provide a coextruded biaxially oriented and transparent polyester film which has at least one matt outer layer and whose production is simple and cost-effective, and which has the good physical properties of known films, causes no disposal problems, and in particular absorbs UV light and has high UV resistance, and also is flame-retardant to DIN 4102, and does not embrittle on exposure to heat.
This object is achieved by means of a coextruded and biaxially oriented polyester film of the generic type mentioned at the outset, the characterizing features of which are that the film comprises at least one UV stabilizer and at least one flame retardant, the flame retardant and, where appropriate, the UV stabilizer being fed by way of masterbatch technology, and has a matt outer layer which comprises a mixture or, respectively, a blend made from two components I and II.
Component I of the mixture or blend is a polyethylene terephthalate homopolymer or polyethylene terephthalate copolymer, or a mixture made from polyethylene terephthalate homo- or copolymers.
Component II of the copolymer or of the mixture or blend is a polymer containing at least one sulfonate group, in particular a condensation product of the following monomers or of their derivatives capable of forming polyesters:
A) from 65 to 95 mol % of isophthalic acid;
B) from 0 to 30 mol % of at least one aliphatic dicarboxylic acid with the formula
HOOC(CH2)nCOOH 
where
n is from 1 to 11;
C) from 5 to 15 mol % of at least one sulfomonomer containing an alkali metal sulfonate group on the aromatic moiety of a dicarboxylic acid;
D) a copolymerizable aliphatic or cycloaliphatic glycol having from 2 to 11 carbon atoms, in the stoichiometric amount necessary to form 100 mol % of condensate;
where each of the percentages given is based on the total amount of the monomers forming component II.
High UV resistance means that sunlight or other UV radiation causes no, or only extremely little, damage to the films, and that the films are therefore suitable for outdoor applications and/or critical indoor applications. In particular, the films should not yellow after a number of years of outdoor use, nor display any embrittlement or cracking of the surface, nor should their mechanical properties deteriorate. High UV resistance therefore means that the film absorbs UV light and does not transmit light until the visible region has been reached.
The good mechanical properties include high modulus of elasticity (EMD greater than 3200 N/mm2; ETD greater than 3500 N/mm2) and also good values for tensile strength at break (in MD greater than 100 N/mm2; in TD greater than 130 N/mm2).
Good orientability includes the capability of the film to give excellent orientation both longitudinally and transversely during its production, without break-offs.
Cost-effective production includes the capability of the raw materials or raw material components needed for producing the flame-retardant film to be dried using conventional industrial dryers. It is important that the raw materials do not cake or undergo thermal degradation. These prior art industrial dryers include vacuum dryers, fluidized-bed dryers, and fixed-bed dryers (tower dryers). These dryers operate at temperatures from 100 to 170xc2x0 C., at which polymers provided with flame retardancy by conventional methods cake and eventually have to be removed by force, making film production impossible. In the case of the vacuum dryer, which has the gentlest drying action, the raw material usually passes through a range of temperatures from about 30 to 130xc2x0 C. at a subatmospheric pressure of 50 mbar. This has to be followed by what is known as post-drying in a hopper at temperatures from 100 to 130xc2x0 C. with a residence time of from 3 to 6 hours. Even here, the known polymers cake to an extreme extent.
No embrittlement on short exposure to heat means that after 100 hours of controlled heating at 100xc2x0 C. in a circulating-air drying cabinet the film has not embrittled and does not have poor mechanical properties.
For the purposes of the present invention, mixtures are mechanical mixtures prepared from the individual components. For this, the individual constituents are generally combined in the form of small-dimensioned compressed moldings, e.g. lenticular or bead-shaped pellets, and mixed with one another mechanically, using a suitable agitator. Another way of producing the mixture is to feed components I and II in pellet form separately to the extruder for the outer layer of the invention, and to carry out the mixing in the extruder and/or in the downstream systems for transporting the melt.
For the purposes of the present invention, a blend is an alloy-like composite of the individual components I and II which can no longer be separated into the initial constituents. A blend has properties like those of a homogeneous material and can therefore be characterized by appropriate parameters.
According to the invention, the film has at least two layers. The layers which it then encompasses are a layer B (=base layer) and the outer layer A of the invention. In one preferred embodiment of the invention, the film has a three-layer structure, and has the outer layer A on one side of the layer B (=base layer) and has another layer C on the other side of the layer B. In this case, the two layers A and C form the outer layers A and C. According to the invention, the UV stabilizer and the flame retardant may be present in the outer layer(s) and/or in the base layer.
The base layer B of the film is preferably composed of at least 70% by weight of a thermoplastic polyester. Polyesters suitable for this purpose are those made from ethylene glycol and terephthalic acid (=polyethylene terephthalate, PET), from ethylene glycol and naphthalene-2,6-dicarboxylic acid (=polyethylene 2,6-naphthalate, PEN), from 1,4-bishydroxymethylcyclohexane and terephthalic acid (=poly-1,4-cyclohexanedimethylene terephthalate, PCDT), or else made from ethylene glycol, naphthalene-2,6-dicarboxylic acid and biphenyl-4,4xe2x80x2-dicarboxylic acid (=polyethylene 2,6-naphthalate dibenzoate, PENBB). Particular preference is given to polyesters at least 90 mol %, preferably at least 95 mol %, of which is composed of ethylene glycol units and terephthalic acid units, or of ethylene glycol units and naphthalene-2,6-dicarboxylic acid units. The remaining monomer units derive from those other aliphatic, cycloaliphatic or aromatic diols and dicarboxylic acids. Other examples of suitable aliphatic diols are diethylene glycol, triethylene glycol, aliphatic glycols of the formula HOxe2x80x94(CH2)nxe2x80x94OH, where n is an integer from 3 to 6 (in particular 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol) and branched aliphatic glycols having up to 6 carbon atoms. Among the cycloaliphatic diols, mention should be made of cyclohexanediols (in particular 1,4-cyclohexanediol). Examples of other suitable aromatic diols have the formula HOxe2x80x94C6H4xe2x80x94Xxe2x80x94C6H4xe2x80x94OH, where X is xe2x80x94CH2xe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94C(CF3)2xe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94SO2xe2x80x94. Bisphenols of the formula HOxe2x80x94C6H4xe2x80x94C6H4xe2x80x94OH are also very suitable.
Other aromatic dicarboxylic acids are preferably benzenedicarboxylic acids, naphthalene dicarboxylic acids (such as naphthalene-1,4- or -1,6-dicarboxylic acid), biphenyl-x,xxe2x80x2-dicarboxylic acids (in particular biphenyl-4,4xe2x80x2-dicarboxylic acid), diphenylacetylene-x,xxe2x80x2-dicarboxylic acids (in particular diphenylacetylene-4,4xe2x80x2-dicarboxylic acid) or stilbene-x,xxe2x80x2-dicarboxylic acids. Among the cycloaliphatic dicarboxylic acids mention should be made of cyclohexanedicarboxylic acids (in particular cyclohexane-1,4-dicarboxylic acid). Among the aliphatic dicarboxylic acids, the C3-C19 g alkanediacids are particularly suitable, and the alkane moiety here may be straight-chain or branched.
One way of preparing the polyesters is the transesterification process. Here, the starting materials are dicarboxylic esters and diols, which are reacted using the customary transesterification catalysts, such as the salts of zinc, of calcium, of lithium, of magnesium or of manganese. The intermediates are then polycondensed in the presence of well-known polycondensation catalysts, such as antimony trioxide or titanium salts. Another equally good preparation method is the direct esterification process in the presence of polycondensation catalysts. This starts directly from the dicarboxylic acids and the diols.
At least one outer layer of the multilayer film of the invention comprises a mixture or, respectively, a blend made from two components I and II and described in more detail below, and, where appropriate, comprises additives.
Component I of the outer layer mixture or of the blend essentially consists of a thermoplastic polyester, in particular the polyester described in more detail for the base layer. A method which has proven successful here for achieving a high degree of mattness is to use a polyester of comparatively low viscosity for component I of the outer layer of the invention. To describe the viscosities of the melts use is made of a modified solution viscosity (SV or xe2x80x9cstandard viscosityxe2x80x9d). The SVs of commercially available polyethylene terephthalates suitable for producing biaxially oriented films are in the range from 500 to 1200. A method which has proven successful for obtaining a high degree of film mattness for the purposes of the present invention is to use polymers whose SV is in the range from 500 to 800, preferably in the range from 500 to 750, particularly preferably in the range from 500 to 700, for component I of the outer layer of the invention.
As stated above, component II of the outer layer mixture is obtained by condensation of the following monomers or of their derivatives capable of forming polymers:
A) isophthalic acid,
B) if appropriate, an aliphatic dicarboxylic acid of the formula
HOOC(CH2)nCOOH 
where
n is in the range from 1 to 11,
C) a sulfomonomer containing an alkali metal sulfonate group on the aromatic moiety of an aromatic dicarboxylic acid, and
D) an aliphatic or cycloaliphatic glycol having from 2 to 11 carbon atoms, in the amount needed to form 100 mol % of condensate.
The total molar equivalents of acid present should be substantially the same as the total equivalents of glycol present.
Examples of dicarboxylic acids suitable as component B) of the copolyesters are malonic, adipic, azelaic, glutaric, sebacic, suberic, succinic and brassylic acid, and also mixtures of these acids or their derivatives capable of forming polyesters. Of the abovementioned acids, sebacic acid is preferred.
Examples of sulfomonomers which contain a metal sulfonate group on the aromatic moiety of an aromatic dicarboxylic acid (component C) are monomers of the following formula: 
where
M is a monovalent cation of an alkali metal,
Z is a trivalent aromatic radical, and
X and Y are carboxy groups or polyester-forming equivalents.
Monomers of this type are described in U.S. Pat. Nos. 3,563,942 and 3,779,993. Examples of monomers of this type are the sodium salts of sulfoterephthalic acid, of 5-sulfoisophthalic acid, of sulfophthalic acid, of 5-(p-sulfophenoxy)isophthalic acid, or of 5-sulfopropoxyisophthalic acid, and similar monomers, and also derivatives of these, such as the dimethyl esters, capable of forming polyesters. M is preferably Na+, Li+, or K+.
The term xe2x80x9cderivatives capable of forming polyestersxe2x80x9d here means reaction participants with groups capable of condensation reactions, in particular transesterification reactions, to form polyester bonds. Groups of this type include carboxy groups. They also include the lower alkyl esters of these, e.g. dimethyl terephthalate, diethyl terephthalate, and numerous other esters, halides, and salts. The acid monomers are preferably used in the form of dimethyl esters, since this permits better control of the condensation reaction.
Examples of glycols suitable as component D) are ethylene glycol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, 1,10-decanediol, cyclohexanedimethanol, and similar substances. It is preferable to use ethylene glycol.
The copolyesters may be prepared by known polymerization techniques. The procedure is generally to combine the acid components with glycol and to heat these in the presence of an esterification catalyst, with subsequent addition of a polycondensation catalyst.
The quantitative ratios of components A, B, C, and D used to prepare the mixtures of the invention have been found to be decisive for obtaining the matt outer layer. For example, at least about 65 mol % of isophthalic acid (component A) has to be present as acid component. Component A is preferably from about 70 to 95 mol % of pure isophthalic acid.
As far as component B is concerned, any acid with the formula mentioned gives satisfactory results, and preference is given to adipic acid, azelaic acid, sebacic acid, malonic acid, succinic acid, glutaric acid, and mixtures of these acids. The desired amount within the range given is preferably from 1 to 20 mol %, based on the acid components of the mixture II, if component B is present in the composition.
The amount of the glycol component present is stoichiometric.
The copolyesters suitable for the purposes of the invention also have an acid value below 10, preferably from 0 to 3, an average molecular weight below about 50,000, and an SV in the range from about 30 to 700, preferably from about 350 to 650.
The ratio (ratio by weight) of the two components I and II of the outer layer mixture or blend may vary within wide limits and depends on the intended use of the multilayer film. The ratio of components I and II is preferably in the range from I:II=10:90 to I:II=95:5, preferably from I:II=20:80 to I:II=95:5, and in particular from I:II=30:70 to I:II=95:5.
According to the invention, the film comprises a UV stabilizer and a flame retardant. The UV stabilizer is advantageously fed by way of what is known as masterbatch technology directly during film production, the concentration of the UV stabilizer here preferably being from 0.01 to 5% by weight, based on the weight of the layer of the crystallizable thermoplastic.
According to the invention, the flame retardant is likewise fed by way of what is known as masterbatch technology directly during film production, the concentration here being from 0.5 to 30% by weight, preferably from 1 to 20% by weight, based on the weight of the layer of the crystallizable thermoplastic.
Light, in particular the ultraviolet content of solar radiation, i.e. the wavelength region from 280 to 400 nm, causes degradation in thermoplastics, the results of which are not only a change in appearance due to color change or yellowing but also an adverse effect on mechanical and physical properties.
The suppression of this photooxidative degradation is of considerable industrial and economic importance, since without it many thermoplastics have drastically reduced scope of application.
The absorption of UV light by polyethylene terephthalates, for example, starts below 360 nm, increasing markedly below 320 nm, and is very pronounced below 300 nm. Maximum absorption occurs at between 280 and 300 nm.
In the presence of oxygen it is mainly chain cleavage which is observed, but without any crosslinking. The predominant photooxidation products in quantity terms are carbon monoxide, carbon dioxide and carboxylic acids. Besides direct photolysis of the ester groups, attention has to be paid to oxidation reactions which proceed via peroxide radicals, again to form carbon dioxide.
In photooxidation of polyethylene terephthalates there can also be cleavage of hydrogen at the position a to the ester groups, giving hydroperoxides and decomposition products of these, and this may be accompanied by chain cleavage (H. Day, D. M. Wiles, J. Appl. Polym. Sci. 16 [1972] p. 203).
UV stabilizers, i.e. light stabilizers which are UV absorbers, are chemical compounds which can intervene in the physical and chemical processes of light-induced degradation. Carbon black and other pigments can give some protection from light, but these substances are unsuitable for transparent films, since they cause discoloration or color change. For transparent, matt films the only suitable compounds are organic or organometallic compounds which give rise to no, or only extremely slight, color or color change in the thermoplastic to be stabilized, i.e. are soluble in the thermoplastic.
UV stabilizers which are suitable light stabilizers are those which absorb at least 70%, preferably 80%, particularly preferably 90%, of the UV light in the wavelength region from 180 to 380 nm, preferably from 280 to 350 nm. These are particularly suitable if they are thermally stable, i.e. do not decompose, nor cause any evolution of gas, in the temperature range from 260 to 300xc2x0 C. Examples of UV stabilizers which are suitable light stabilizers are 2-hydroxybenzophenones, 2-hydroxybenzotriazoles, organonickel compounds, salicylic esters, cinnamic ester derivatives, resorcinol monobenzoates, oxanilides, hydroxybenzoic esters, sterically hindered amines and triazines, preference being given to the 2-hydroxybenzotriazoles and the triazines.
The film of the invention comprises at least one flame retardant, fed by way of what is known as masterbatch technology directly during film production, the concentration of the flame retardant being in the range from 0.5 to 30.0% by weight, preferably from 1.0 to 20.0% by weight, based on the weight of the layer of the crystallizable thermoplastic. The ratio by weight of flame retardant to thermoplastic usually maintained when producing the masterbatch is in the range from 60:40 to 10:90.
Typical flame retardants include bromine compounds, chloroparaffins and other chlorine compounds, antimony trioxide, and alumina trihydrates, but the halogen compounds are disadvantageous due to the halogen-containing by-products produced. Another extreme disadvantage is the low lightfastness of any film provided with these, and also the evolution of hydrogen halides in the event of a fire.
Examples of suitable flame retardants used according to the invention are organophosphorus compounds, such as carboxyphosphinic acids, anhydrides of these, and dimethyl methylphosphonate. It is important for the invention that the organophosphorus compound is soluble in the thermoplastic, since otherwise compliance with the required optical properties is lacking.
Since the flame retardants generally have some degree of susceptibility to hydrolysis, the additional use of a hydrolysis stabilizer can be advisable.
The hydrolysis stabilizers generally used are phenolic stabilizers, alkali metal/alkaline earth metal stearates, and/or alkali metal/alkaline earth metal carbonates, in amounts of from 0.01 to 1.0% by weight. The amount used of phenolic stabilizers is preferably from 0.05 to 0.6% by weight, in particular from 0.15 to 0.3% by weight, and their molar mass is preferably more than 500 g/mol. Pentaerythrityl tetrakis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate or 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene is particularly advantageous.
In one particularly preferred embodiment, the film of the invention comprises from 1 to 20% by weight of a flame retardant, such as dimethyl methylphosphonate and from 0.01 to 5.0% by weight of 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-hexyloxyphenol of the formula 
or from 0.01 to 5.0% by weight of 2,2-methylenebis(6-(2H-benzotriazol-2-yl)-4-(1,1,2,2-tetramethylpropyl)-phenol) of the formula 
In one preferred embodiment, use may also be made of a mixture of these two UV stabilizers, or of a mixture of at least one of these two UV stabilizers with other UV stabilizers, the total concentration of light stabilizer preferably being from 0.01 to 5.0% by weight, based on the weight of crystallizable polyethylene terephthalate.
Besides the flame retardant fed by way of masterbatch technology and the UV stabilizer, the base layer and/or the outer layer(s) may also comprise conventional additives, such as stabilizers and antiblocking agents. They are advantageously added to the polymer or polymer mixture before melting begins. Examples of stabilizers used are phosphorus compounds, such as phosphoric acid or phosphoric esters. Typical antiblocking agents (also termed pigments in this context) are inorganic and/or organic particles, such as calcium carbonate, amorphous silica, talc, magnesium carbonate, barium carbonate, calcium sulfate, barium sulfate, lithium phosphate, calcium phosphate, magnesium phosphate, aluminum oxide, lithium fluoride, the calcium, barium, zinc, or manganese salts of the dicarboxylic acids used, carbon black, titanium dioxide, kaolin, or crosslinked polymer particles, such as polystyrene particles or acrylate particles.
Additives selected may also be mixtures of two or more different antiblocking agents, or mixtures of antiblocking agents of the same composition, but with different particle sizes. The usual concentration of the particles may be added to each of the layers, e.g. in the form of a glycolic dispersion during polycondensation or by way of masterbatches during extrusion. Pigment concentrations which have proven particularly suitable are from 0.0001 to 10% by weight. Addition of these particles to the outer layer gives another advantageous opportunity of varying the degree of mattness of the film. Increase in pigment concentration is generally associated with an increase in the degree of mattness of the film. An example of a detailed description of antiblocking agents is found in EP A 0 602 964.
The present invention also provides a process for producing this film. It encompasses
a) producing a film from base and outer layer(s) by coextrusion,
b) biaxial orientation of the film, and
c) heat-setting of the oriented film.
It is important for the invention that the masterbatch which comprises the flame retardant and, where appropriate, the hydrolysis stabilizer is precrystallized or predried. This predrying includes gradual heating of the masterbatch at subatmospheric pressure (from 20 to 80 mbar, preferably from 30 to 60 mbar, in particular from 40 to 50 mbar), with stirring, and, where appropriate, post-drying at constant increased temperature, again at subatmospheric pressure. The masterbatch is preferably charged at ambient temperature from a feed vessel in the required blend together with the polymers of the base and/or outer layers and, where appropriate, with other raw material components batchwise into a vacuum dryer which during the course of the drying or residence time passes through a temperature profile of from 10 to 160xc2x0 C., preferably from 20 to 150xc2x0 C., in particular from 30 to 130xc2x0 C. During a residence time of about 6 hours, preferably 5 hours, in particular 4 hours, the raw material mixture is stirred at from 10 to 70 rpm, preferably from 15 to 65 rpm, in particular from 20 to 60 rpm. The resultant precrystallized or predried raw material mixture is post-dried in a downstream vessel, likewise evacuated, at from 90 to 180xc2x0 C., preferably from 100 to 170xc2x0 C., in particular from 110 to 160xc2x0 C., for from 2 to 8 hours, preferably from 3 to 7 hours, in particular from 4 to 6 hours.
To produce the outer layer of the invention, it is advantageous to feed pellets of mixture component I and pellets of mixture component II in the desired mixing ratio directly to the extruder, where appropriate together with the flame retardant masterbatch which has been predried. It has proven advantageous for extrusion of the matt outer layer of the invention to use a twin-screw extruder, e.g. as described in EP 0 826 478. The materials can be melted and extruded at about 300xc2x0 C. with a residence time of about 5 min. Under these conditions, transesterification reactions can proceed in the extruder, and these can form other copolymers from the homopolymers and the copolymers.
The polymers for the base layer are advantageously fed by way of another extruder, together with the flame retardant masterbatch, which has been predried. Any foreign bodies or contamination present may be filtered out from the polymer melt prior to extrusion. The melts are then extruded through a coextrusion die to give flat melt films and laminated to one another. The multilayer film is then drawn off and solidified with the aid of a chill roll and, where appropriate, other rolls.
The biaxial orientation is generally carried out sequentially or simultaneously.
In sequential stretching, it is preferable to orient first longitudinally (i.e. in machine direction) and then transversely (i.e. perpendicularly to machine direction). This leads to orientation of the molecular chains. The longitudinal orientation can be carried out with the aid of two rolls running at different speeds corresponding to the desired stretching ratio. For transverse orientation use is generally in the range made of an appropriate tenter frame. In simultaneous stretching the film is simultaneously stretched longitudinally and transversely in a tenter frame.
The temperature at which the orientation is carried out may vary within a relatively wide range and depends on the desired properties of the film. The longitudinal stretching is generally carried out at from 80 to 130xc2x0 C. and the transverse stretching at from 90 to 150 C. The longitudinal orientation ratio is generally in the range from 2.5:1 to 6:1, preferably from 3:1 to 5.5:1. The transverse stretching ratio is generally in the range from 3.0:1 to 5.0:1, preferably from 3.5:1 to 4.5:1. If desired, the transverse stretching may be followed by another longitudinal orientation, and even by another transverse orientation.
In the heat-setting which follows, the film is held at a temperature of from 150 to 250xc2x0 C. for from 0.1 to 10 s. The film is then wound up in the usual way.
One or both surfaces of the film may therefore have a coating. The thickness of this coating on the finished film is from 5 to 100 nm, preferably from 20 to 70 nm, in particular from 30 to 50 nm. It is preferably applied in-line, i.e. during the film-production process, advantageously prior to transverse orientation. Application by reverse gravure roll coatings is particularly preferred, and this process permits extremely uniform application of the coating at the layer thickness mentioned. The coatings are appliedxe2x80x94preferably by aqueous methodsxe2x80x94as solutions, suspensions, or dispersions, and provide the film surface with additional functionality, for example making the film sealable, printable, metallizable, sterilizable, antistatic, or improving its aroma barrier for example, or permitting its adhesion to materials which would not otherwise adhere to the film surface (e.g. photographic emulsions). Examples of substances or compositions which provide additional functionality are:
Acrylates, as described by way of example in WO 94/13476, ethylene-vinyl alcohols, PVDC, waterglass (Na2SiO4), hydrophilic polyesters (PET/IPA polyesters containing the sodium salt of 5-sulfoisophthalic acid, for example as described in EP-A-0 144 878 or U.S. Pat. No. 4,252,885 or EP-A-0 296 620), vinyl acetates, for example as described in WO 94/13481, polyvinyl acetates, polyurethanes, the alkali metal or alkaline earth metal salts of C10-C18 fatty acids, and butadiene copolymers with acrylonitrile or methyl methacrylate, methacrylic acid, acrylic acid, or esters thereof.
The compositions or substances mentioned are applied in the form of dilute, preferably aqueous, solution, emulsion, or dispersion to one or both surfaces of the film. The solvent is then evaporated. If the coatings are applied in-line prior to transverse stretching, the heat treatment during transverse stretching and subsequent heat-setting is usually sufficient to evaporate the solvent and dry the coating. The dried coatings then have the abovementioned desired layer thicknesses.
The films may also be coatedxe2x80x94preferably in an off-line processxe2x80x94with metals, such as aluminum, or with ceramic materials, such as SiOx or AlxOy. This in particular improves their gas-barrier properties.
The polyester film of the invention preferably also comprises a second outer layer C. The structure, thickness, and composition of the second outer layer may be selected irrespective of the other outer layer present, and the second outer layer may likewise comprise the abovementioned polymers, the UV stabilizer, the flame retardant, or polymer mixtures for the base layer or the first outer layer of the invention, but these do not have to be identical with those of the first outer layer. The second outer layer may also comprise other commonly used outer layer polymers, while these may also be provided with UV stabilizer and/or the flame retardant.
If desired, there may also be an intermediate layer between the base layer and the outer layer(s). The intermediate layer may be composed of the polymers described for the base layer. In one particularly preferred embodiment, it is composed of the polyester used for the base layer. It may also comprise the conventional additives described, and the flame retardant and/or the UV stabilizer. The thickness of the intermediate layer is generally above 0.3 xcexcm, preferably in the range from 0.5 to 15 xcexcm, in particular from 1.0 to 10 xcexcm.
The thickness of the outer layer(s) is generally above 0.1 xcexcm, preferably in the range from 0.2 to 5 xcexcm, in particular from 0.2 to 4 xcexcm, and the thicknesses of the outer layers may be identical or different.
The total thickness of the polyester film of the invention may vary within wide limits and depends on the intended application. It is preferably from 4 to 500 xcexcm, in particular from 5 to 450 xcexcm, with preference from 6 to 300 xcexcm, the base layer preferably making up from about 40 to 90% of the total thickness.
It is entirely surprising that the use of the abovementioned UV stabilizers in films led to the desired result. The skilled worker would probably first have attempted to achieve UV resistance by using an antioxidant, but would have discovered that on weathering the film rapidly yellows.
On using conventional UV stabilizers, the skilled worker would have found that
the UV stabilizer has inadequate thermal stability and at temperatures of from 200 to 240xc2x0 C. decomposes and evolves gases
large amounts of UV stabilizer would have had to have been incorporated (from about 10 to 15% by weight) to absorb the UV light and prevent damage to the film.
At these high concentrations, the skilled worker would have found that the film becomes yellow just after it has been produced, with Yellowness Index deviations (YI) around 25. The mechanical properties of the film would also have been found to be adversely affected. Orientation would have produced exceptional problems, such as
break-offs due to unsatisfactory strength, i.e. modulus of elasticity too low;
die deposits, causing profile variations;
roller deposits from the UV stabilizer, causing immpairment of optical properties (poor haze, defective adhesion, non-uniform surface);
deposits in stretching frames or heat-setting frames, dropping onto the film.
It was therefore more than surprising that even low concentrations of the UV stabilizer of the invention achieve excellent UV protection. It was very surprising that, together with this excellent UV protection:
within the accuracy of measurement, the Yellowness Index of the film is unchanged from that of an unstabilized film;
there were no evolution of gases, no die deposits, and no frame condensation, and the film therefore has excellent optical properties and excellent profile and layflat;
the UV-resistant film has excellent stretchability, and can therefore be produced in a reliable and stable manner on high-speed film lines at speeds of up to 420 m/min.
It was also surprising that a flame-retardant film with the required property profile can be produced cost-effectively and without caking in the dryer by using masterbatch technology and suitable predrying and/or precrystallization and, where appropriate, small amounts of a hydrolysis stabilizer, and that the film does not embrittle by exposure to heat and does not break when folded.
It is moreover very surprising that it is also possible to reuse the recycled material (regrind) produced from the films without adversely affecting the Yellowness Index of the film.
The film of the invention can readily be recycled without pollution of the environment, and is therefore suitable for use as short-lived advertising placards, in the construction of exhibition stands, or for other short-lived promotional items, where fire protection and UV absorption are desired.
Another advantage is that the production costs of the film of the invention are only slightly higher than those for a film made from standard polyesters. The other properties of the film of the invention relevant to its processing and use are substantially unchanged or indeed have been improved. In addition, it has been ensured that during production of the film it is possible to reuse the regrind in a proportion of up to 50% by weight, preferably from 10 to 50% by weight, based in each case on the total weight of the film, without any significant resultant adverse effect on the physical properties of the film.
In summary, the film of the invention has flame retardancy to DIN 4102 (construction materials classes B1 and B2), is impermeable to UV light, is highly UV resistant, has low gloss, in particular low gloss on film surface A, and has comparatively low haze. It moreover has good winding and processing performance. It is also worthy of mention that the outer layer of the invention has good writability with respect to ballpoint pen, felt-tip pen, or fountain pen.
The gloss of film surface A is lower than 70. In one preferred embodiment the gloss of this side is less than 60, and in one particularly preferred embodiment less than 50. This surface of the film is therefore particularly effective for promotional purposes.
In addition, the film complies with construction materials classes B1 and B2 to DIN 4102 Part 1 and Part 2 and shows no embrittlement on exposure to heat. The Yellowness Index (YI) of the film of the invention is not higher than that of a standard film. The film begins to transmit light or radiation at  greater than 360 nm, i.e. the film absorbs the harmful UV radiation. A non-UV-resistant film transmits radiation from as low as 280 nm, i.e. the UV light in the wavelength region from 280 to 360 nm is not absorbed but transmitted by the film.
The haze of the film of the invention is smaller than 40%. In one preferred embodiment, the haze of the film is less than 35%, and in one particularly preferred embodiment less than 30%. The comparatively low haze of the film (compared with a matt monofilm, see comparative example) means that the film can, for example, be reverse-printed, or viewing windows can be incorporated through which, for example, the contents can be clearly discerned.
The combination of exceptional properties gives the film of the invention excellent suitability for a wide variety of applications, for example for interior decoration, for construction of exhibition stands or for exhibition requisites, as displays, for placards, for protective glazing of machinery or vehicles, in the lighting sector, in the fitting out of shops or of stores, as a promotional item, or laminating medium, for greenhouses, roofing systems, exterior cladding, protective coverings, applications in the construction sector, and illuminated advertising profiles, blinds, and electrical applications.
Other application sectors are its use for producing labels, as a release film, or as a hot-stamping foil, or in-mold labeling.
The table below (Table 1) gives again the most important film properties of the invention.
The methods used to characterize the polymers and the films were as follows:
Test Methods
DIN=Deutsches Institut fxc3xcr Normung
ISO=International Organization for Standardization
ASTM=American Society for Testing and Materials
SV (DCA), IV (DVE)
Standard viscosity SV (DCA) is measured in dichloroacetic acid by a method 30 based on DIN 53726.
Intrinsic viscosity (IV) is calculated as follows from standard viscosity
IV(DCA)=6.67xc2x710xe2x88x924SV(DCA)+0.118 
Coefficient of Friction
Coefficient of friction is determined to DIN 53 375, 14 days after production.
Surface Tension
Surface tension was determined by what is known as the ink method (DIN 53 364).
Haze
Haze of the film was measured to ASTM D1003-52. The Hxc3x6lz haze measurement was made by a method based on ASTM D1003-52, but in order to utilize the ideal measurement range was measured on four mutually superimposed laps of film, and a 1xc2x0 slit diaphragm was used instead of a 4xc2x0 pinhole.
Gloss
Gloss was determined to DIN 67 530. Reflectance was measured, this being an optical value characteristic of a film surface. Based on the standard ASTM D523-78 and ISO 2813, the angle of incidence was set at 20 or 60. A beam of light hits the flat test surface at the set angle of incidence and is reflected and/or scattered by the surface. A proportional electrical variable is displayed representing light rays hitting the photoelectric detector. The value measured is dimensionless and must be stated together with the angle of incidence.
Roughness
The roughness Ra of the film was determined to DIN 4768 with a cutoff of 0.25 mm.
Mechanical Properties
Modulus of elasticity and tensile strength at break, and tensile strain at break, were measured longitudinally and transversely to ISO 527-1-2.
Weathering (Bilateral), UV Resistance
UV resistance was tested as follows to the ISO 4892 test specification:
Numerical values of  less than 0.3 can be disregarded and mean that no significant color change has occurred.
Yellowness Index
Yellowness Index (YI) is the deviation from colorlessness in the xe2x80x9cyellowxe2x80x9d direction and is measured to DIN 6167. Yellowness Indices (YIs) less than 5 are not visually detectable.
Each of the examples and comparative examples below concerns transparent films of varying thickness, produced on the extrusion line described.
All of the films were weathered bilaterally, each for 1000 hours per side, to the test specification ISO 4892 using the Atlas C165 Weather-Ometer, and then tested for mechanical properties, Yellowness Index (YI), surface defects, light transmittance, and gloss.
The examples below provide illustration of the invention.